1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with top nozzle-mounted replacement guide pin assemblies for alignment of fuel assemblies with an upper core plate.
2. Description of the Prior Art
A pressurized water nuclear reactor (PWR) typically includes a nuclear reactor core composed of a large number of nuclear fuel assemblies supported between vertically spaced upper and lower core plates. The upper and lower core plates are supported by a core support barrel which surrounds the reactor core. The fuel assemblies are arranged in vertical orientations and in side-by-side relationship.
A typical fuel assembly includes a plurality of fuel rods held in a stationary array by a skeleton of the fuel assembly. The fuel assembly skeleton is composed of a plurality of control rod guide thimbles disposed between and extending parallel to the fuel rods, a plurality of grids mounted to and axially spaced along the guide thimbles and supporting the array of fuel rods, and top and bottom nozzles disposed adjacent to and attached at opposite ends of the guide thimbles.
The top nozzle of the fuel assembly is an enclosure or housing formed by a transversely extending lower adapter plate and an upper annular flange with an upstanding sidewall extending between and integrally interconnecting the adapter plate and flange at their respective peripheries. The lower adapter plate is provided with one plurality of holes to permit the flow of coolant upward through the top nozzle and another plurality of holes to receive the upper ends of the guide thimbles which, in turn, are attached to the lower adapter plate. The upper annular flange defines a central top opening of the top nozzle within which may be disposed a rod cluster control assembly (RCCA). The RCCA contains a plurality of control rods which can be inserted into and withdrawn from the guide thimbles of the fuel assembly through the set of holes in the lower adapter plate receiving the upper ends of the guide thimbles. The upper annular flange also has two holes disposed in two of the diagonal corners of the top nozzle which mate with guide pins mounted in the upper core plate.
The guide pins disposed between the upper core plate and top nozzle of each fuel assembly provide proper alignment and engagement of the fuel assembly with the upper core plate so that the guide thimbles of the fuel assembly will extend vertically in alignment with the control rods for receiving the control rods from above. In the event a guide pin should incur damage affecting the proper alignment of the fuel assembly, this may adversely impact the capability to move the control rods into the guide thimbles of the fuel assembly. Thus, it would be highly desirable to be able to take steps to replace the damaged guide pin.
However, attempts to perform in-situ guide pin repair and replacement have not been pursued in view of the following obstacles which are a result of prior art guide pin designs. First, all procedures must be performed remotely underwater due to unacceptably high levels of radiation. Second, duplication of original equipment guide pin shank preload via shrink fitting is practically impossible to achieve since it was carried out by submersion of the pin shank into liquid nitrogen. Third, access to accommodate guide pin nut removal is difficult since the nut is recessed in the top side of the upper core plate. Fourth, extensive tooling design and development is necessary to qualify tooling and repair procedures.
Consequently, there is a need for improvements in the design of the guide pins for avoiding the abovedescribed drawbacks.